The present invention relates in general to substrate manufacturing technologies and in particular to methods and apparatus for in situ substrate temperature monitoring.
In the processing of a substrate, e.g., a semiconductor substrate or a glass panel such as one used in flat panel display manufacturing, plasma is often employed. As part of the processing of a substrate for example, the substrate is divided into a plurality of dies, or rectangular areas, each of which will become an integrated circuit. The substrate is then processed in a series of steps in which materials are selectively removed (etching) and deposited (deposition) in order to form electrical components thereon.
In an exemplary plasma process, a substrate is coated with a thin film of hardened emulsion (i.e., such as a photoresist mask) prior to etching. Areas of the hardened emulsion are then selectively removed, causing parts of the underlying layer to become exposed. The substrate is then placed in a plasma processing chamber on a substrate support structure comprising a mono-polar or bi-polar electrode, called a chuck or pedestal. Appropriate etchant source are then flowed into the chamber and struck to form a plasma to etch exposed areas of the substrate.
Among the set of process variables that can be adjusted to optimize the plasma process are gas composition, gas phase, gas flow, gas pressure, RF power density, voltage, magnetic field strength, and substrate temperature. Although in theory it may be beneficial to optimize each variable to each processing step, in practice it is often difficult to achieve.
For example, in order to enhance the uniformity of plasma processing of a substrate in a plasma processing apparatus, it is desirable to control the temperature at exposed surfaces of the substrate at which etching occurs, on which material is deposited (e.g., by a CVD or PVD technique), and/or at which photoresist is removed. For example, if a substrate's temperature rises above a certain temperature, substrate damage (e.g., photoresist damage) can occur, and temperature-dependent chemical reactions can be altered. Substrate temperature may also significantly affect plasma selectivity by changing the deposition rate of polymeric films, such as poly-floro-carbon on the substrate surface. Careful monitoring may minimize variation, allow a wider process window for other parameters, and improve process control. However, in practice it may be difficult to directly determine temperature without affecting the plasma process.
In a common temperature measuring technique, a thermocouple is coupled to a substrate. In order to measuring the difference between its own temperature and that of the substrate, a thermocouple must make mechanical contact with the sample. However, in many cases, it is not acceptable or feasible to touch the surface of the substrate during processing. In addition, it is often hard to thermally isolate the thermocouple. That is, the measured temperature may not only comprise radiation generated by the substrate, but may also include thermal energy propagated by other structures in proximity to the thermocouple, such as the chuck.
Another set of techniques involves the use of electromagnetic probes. For example, an electromagnetic pyrometer computes temperature from the intensity of the substrate's emitted radiation (e.g., photoluminescence). In general, a substrate may absorb electromagnetic radiation of some frequency and then emit radiation at another frequency corresponding to the substrates specific structure, composition and quality. However, since other heated structures within the plasma processing system may also produce radiation at the same frequency, isolating the substrate measurement from the background may be problematic. Furthermore, since most optical measurement techniques are sensitive to physical variations between substrates (e.g., doping levels, circuit geometry, backside films, etc.), it is often difficult to determine an absolute temperature. That is, the zero point temperature cannot be readily known. Additionally, for substrate temperatures less than approximately 100° C. (commonly used in plasma processing) the radiated energy may be very small and difficult to detect.
In another electromagnetic technique, an interferometer is used to measure a change in substrate thickness due to absorbed thermal energy. Generally, an interferometer measures a physical displacement by sensing a phase difference of an electromagnetic beam reflected between two surfaces. In a plasma processing system, an electromagnetic beam may be transmitted at a frequency for which the substrate is translucent, and positioned at an angle beneath the substrate. A first portion of the beam may then reflect on the substrate's bottom surface, while the remaining portion of the beam may reflect on the substrate's top surface. However, as before, it is often difficult to determine an absolute temperature since most optical measurement techniques are sensitive to physical variations between substrates. Subsequently, a change to substrate thickness may only generally be correlated only to a corresponding change in temperature.
Still another electromagnetic technique is DRS, or diffuse reflectance spectroscopy. DRS determines the temperature of a semiconductor substrate by spectroscopic analysis of diffusely reflected (or transmitted) incident white light. However, this technique relies on measuring a relatively weak diffusely scattered light signal. Any process that makes the substrate opaque to light renders the DRS signal too low to make accurate temperature measurements (i.e., coating the substrate with a metal, coating the substrate with an absorbing layer, free carrier absorption, etc.). Furthermore, like other electromagnetic techniques, is also suffers from a sensitivity to substrate variations previously described.
Referring now to FIG. 1, a simplified diagram of a plasma processing system 100 is shown. Generally, an appropriate set of gases is flowed into chamber 102 through an inlet 108 from gas distribution system 122. These plasma processing gases may be subsequently ionized to form a plasma 110, in order to process (e.g., etch or deposition) exposed areas of substrate 114, such as a semiconductor substrate or a glass pane, positioned on an electrostatic chuck 116.
Gas distribution system 122 is commonly comprised of compressed gas cylinders 124a–f containing plasma processing gases (e.g., C4F8, C4F6, CHF3, CH2F3, CF4, HBr, CH3F, C2F4, N2, O2, Ar, Xe, He, H2, NH3, SF6, BCl3, Cl2, WF6 etc.). Gas cylinders 124a–f may be further protected by an enclosure 128 that provides local exhaust ventilation. Mass flow controllers 126a–f are commonly a self-contained devices (consisting of a transducer, control valve, and control and signal-processing electronics) commonly used in the semiconductor industry to measure and regulate the mass flow of gas to the plasma processing system.
Induction coil 131 is separated from the plasma by a dielectric window 104, and generally induces a time-varying electric current in the plasma processing gases to create plasma 110. The window both protects induction coil from plasma 110, and allows the generated RF field to penetrate into the plasma processing chamber. Further coupled to induction coil 131 at leads 130a–b is matching network 132 that may be further coupled to RF generator 138. Matching network 132 attempts to match the impedance of RF generator 138, which typically operates at 13.56 MHz and 50 ohms, to that of the plasma 110.
Generally, some type of cooling system is coupled to the chuck in order to achieve thermal equilibrium once the plasma is ignited. The cooling system itself is usually comprised of a chiller that pumps a coolant through cavities within the chuck, and helium gas fed between the chuck and the substrate. The heat generated in the wafer during plasma processing then flow through the helium, into the chuck, and out to a remotely located heat exchanger unit.
However, although substrate temperature in generally stabilized within a range, its exact value is commonly unknown. Furthermore, since the substrate temperature may not be directly measured, optimizing the recipe may be difficult For example, in creating a set of plasma processing steps for the manufacture of a particular substrate, a corresponding set of process variables, or recipe, may be established. Temperature repeatability between substrates is often important, since many plasma processing recipes may also require temperature variation to be on the order of a few tenths of degree C.
In a typical plasma processing system, a substrate temperature may be determined by calculating plasma power deposited onto the substrate and the heat transfer coefficient derived from the He pressure and the chuck surface conditions. However, since the cooling system may also be operating in an open-loop fashion, subsequent heat flow variations may cause the substrate temperature to vary outside narrow recipe parameters.
Furthermore, the physical structure of the plasma processing chamber, itself, may change. For example, during chamber cleaning, process pollutants may be removed from the plasma processing system by striking the plasma without the substrate. However, during this cleaning process, as the chuck is no longer shielded by the substrate, it is subsequently etched. As the process is repeated, the substrate's surface roughness increases, subsequently modifying its heat transfer efficiency. Eventually, the recipe's parameters are invalidated. Since it is often impractical to determine when this point is exactly reached, the chuck is generally replaced after a certain amount of operational hours, which in practice is normally only a fraction of its useful life. This can both increase productions costs, since an expensive chuck may be needless replaced, and reduces yield, since the plasma processing system must be taken offline for several hours to replace the chuck.
In addition, recipe parameters may also need to be adjusted. For example, a process engineer may wish to increase the level of passivation during plasma processing. Furthermore, an otherwise identical piece of fabrication equipment may be installed at a different time, or is used to a different degree, and its maintenance cycle does not necessarily match that of the others. The recipe parameters may need to be adjusted when moving the process to a newer version of the plasma processing system, or when transferring the process to a plasma processing system that can process a larger substrate size (e.g., 200 mm to 300 mm). Ideally, it would be beneficial to maintain the same recipe parameters (e.g., chemistry, power, and temperature). However, since substrate temperature has been generally inferred and not measured, the process may need to be substantially adjusted through trial and error in order to achieve a similar production profile.
In view of the foregoing, there are desired improved methods and apparatus for in situ substrate temperature monitoring.